U.S. patent application number 12/931999 was filed with the patent office on 2011-09-01 for low resistivity contact to iron-pnictide superconductors.
This patent application is currently assigned to Iowa State University Research Foundation, Inc.. Invention is credited to Sergey Bud'ko, Paul Canfield, Ni Ni, Ruslan Prozorov, Makariy Tanatar.
Application Number | 20110212841 12/931999 |
Document ID | / |
Family ID | 44505586 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110212841 |
Kind Code |
A1 |
Tanatar; Makariy ; et
al. |
September 1, 2011 |
Low resistivity contact to iron-pnictide superconductors
Abstract
Method of making a low resistivity electrical connection between
an electrical conductor and an iron pnictide superconductor
involves connecting the electrical conductor and superconductor
using a tin or tin-based material therebetween, such as using a tin
or tin-based solder. The superconductor can be based on doped
AFe.sub.2As.sub.2, where A can be Ca, Sr, Ba, Eu or combinations
thereof for purposes of illustration only.
Inventors: |
Tanatar; Makariy; (Ames,
IA) ; Prozorov; Ruslan; (Ames, IA) ; Ni;
Ni; (Ames, IA) ; Bud'ko; Sergey; (Ames,
IA) ; Canfield; Paul; (Ames, IA) |
Assignee: |
Iowa State University Research
Foundation, Inc.
|
Family ID: |
44505586 |
Appl. No.: |
12/931999 |
Filed: |
February 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61338832 |
Feb 24, 2010 |
|
|
|
Current U.S.
Class: |
505/236 ;
174/84R; 228/262.9; 505/300 |
Current CPC
Class: |
H01R 13/03 20130101;
H01R 4/68 20130101; H01L 39/02 20130101 |
Class at
Publication: |
505/236 ;
505/300; 228/262.9; 174/84.R |
International
Class: |
H01L 39/24 20060101
H01L039/24 |
Goverment Interests
CONTRACTUAL ORIGIN OF THE INVENTION
[0002] This invention was made with government support under
Contract No. DE-AC02-07CH11358 from the Department of Energy. The
government has certain rights in the invention.
Claims
1. A method of making an electrical connection between a metallic
electrical conductor and an iron pnictide superconductor,
comprising connecting the electrical conductor and superconductor
using a tin or tin-based material disposed therebetween.
2. The method of claim 1 wherein the tin or tin-based material is a
solder that is solidified between the electrical conductor and the
superconductor.
3. The method of claim 1 wherein the superconductor is represented
by
(A.sub.1-xD1.sub.x)(Fe.sub.1-yD2.sub.y).sub.2(As.sub.1-zD3.sub.z).sub.2
wherein A comprises an alkali earth element selected from the group
consisting of Ca, Sr, Ba, Eu, and combinations thereof and x is 0
to 1, y is 0 to 0.4, and z is 0 to 0.6 and wherein D1 is selected
from the group consisting of Na, K, Rb, Cs and combinations
thereof, D2 is selected from the group consisting of Co, Ni, Pd,
Rh, Ru, Pt and combinations thereof, and D3 is selected from the
group consisting of P, Te, S, Se, Sb, Bi and combinations
thereof.
4. The method of claim 3 wherein y is 0 to 0.2.
5. The method of claim 1 wherein the tin or tin-based material
comprises ultrahigh purity tin that is 99.999% by weight tin.
6. The method of claim 1 wherein the tin or tin-based material
comprises an alloy of tin and lead, an alloy of tin and copper, an
alloy of tin and silver, or a ternary alloy of
tin-silver-copper.
7. An electrical connection between a metallic electrical conductor
and an iron pnictide superconductor wherein the connection
comprises a tin or tin-based material disposed between the
electrical conductor and the superconductor.
8. The connection of claim 7 wherein the material comprises a
solder that is solidified between the electrical conductor and the
superconductor.
9. The connection of claim 7 wherein the superconductor is
represented by
(A.sub.1-xD1.sub.x)(Fe.sub.1-yD2.sub.y).sub.2(As.sub.1-zD3.sub.z).sub.2
wherein A comprises an alkali earth element selected from the group
consisting of Ca, Sr, Ba, Eu, and combinations thereof and x is 0
to 1, y is 0 to 0.4, and z is 0 to 0.6 and wherein D1 is selected
from the group consisting of Na, K, Rb, Cs and combinations
thereof, D2 is selected from the group consisting of Co, Ni, Pd,
Rh, Ru, Pt and combinations thereof, and D3 is selected from the
group consisting of P, Te, S, Se, Sb, Bi and combinations
thereof.
10. The connection of claim 9 wherein y is 0 to 0.2.
11. The connection of claim 7 wherein the material comprises
ultrahigh purity tin that is 99.999% by weight tin.
12. The connection of claim 7 wherein the material comprises an
alloy of tin and lead.
13. The connection of claim 7 wherein the material comprises an
alloy of tin and silver.
14. The connection of claim 7 wherein the material comprises an
alloy of tin and copper.
15. The connection of claim 7 wherein the material comprises an
alloy of tin, copper and silver.
16. The connection of claim 7 wherein the material comprises a
tin-based alloy that includes a minor amount of an element selected
from the group consisting of Bi, Al, In, Pb, Ag, Au, Cu, Cd, Sb,
and Zn.
Description
[0001] This application claims benefits and priority of U.S.
provisional application Ser. No. 61/338,832 filed Feb. 24, 2010,
the disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to a low resistivity
electrical contact to a superconductor and, more particularly, to a
low resistivity electrical contact to an iron-pnictide
superconductor and to a contact making method.
BACKGROUND OF THE INVENTION
[0004] Superconductors are materials which carry electrical current
without dissipation. However, feeding electrical current into a
superconductor generates heat dissipation in the contacts and
degrades maximum attainable current value. This degradation can be
minimized by creating electrical contacts with low resistance.
[0005] The superconducting state is destroyed if the parameters of
the experiment exceed some critical values. These are critical
temperature T.sub.c and critical magnetic fields, the higher of
which called the upper critical field, H.sub.c2 destroys
superconducting state completely. The maximum density of current
which the superconductor can carry without dissipation is called
critical current density J.sub.c. This value, in addition to
temperature and magnetic field, depends on both intrinsic
properties of materials used and the form of their preparation.
[0006] There is an ongoing effort to find superconductors with high
intrinsic critical parameters. Discovery of superconductors with
high transition temperatures, well exceeding the boiling point of
liquid hydrogen, 20 K, opens the way for using high temperature
superconductors for a variety of application. Just a few
superconductors are known with transition temperatures on the order
of 40 K, offering prospective for technological applications at 20
K. These include numerous compounds based on copper oxides and
generally referred to as superconducting cuprates (T.sub.c up to
135 K at ambient pressure) and MgB.sub.2 (T.sub.c=39K). A new
family of materials based on iron-arsenide compounds known as iron
pnictides, which are generally represented by AFe.sub.2As.sub.2
where A can be Ca, Sr, Ba or combinations thereof, offer a number
of superior material parameters as compared to the cuprates. These
include low anisotropy of the upper critical fields, high values of
the upper critical field and small anisotropy of the
superconducting critical currents.
[0007] Superconductivity can be induced in the iron pnictides by
partial chemical (elemental) substitution (doping) of either of the
three elemental constituents (A, Fe, As) and/or by application of
pressure. Substituted iron pnictides can be represented by
(A.sub.1-xD1.sub.x)(Fe.sub.1-yD2.sub.y).sub.2(As.sub.1-zD3.sub.z).sub.2,
where A stands for alkali earth elements Ca, Sr, Ba, Eu or their
combinations, and various elements can be added as dopants D1, D2
and D3.
[0008] Superconducting materials based on K-, P-, Co-, or Ni-doped
BaFe.sub.2As.sub.2, with high transition temperatures to the
superconducting state (above 20 K) were discovered in mid-2008 and
their electrical properties were tested using commonly accepted
in-laboratory electrical contact making techniques that involved
attaching electrical conductor wires (e.g. silver wires) to the
superconductor using silver epoxies or silver paint. However, these
electrical connections were characterized by high contact
resistance on the order of 10.sup.-3 ohm-cm.sup.2, and are not
suitable for high current applications.
[0009] Feeding of electrical current into the superconductor
generates heat dissipation in the contacts and degrades maximum
attainable current value. This degradation can be minimized by
creating contacts with low resistance. However, the selection of
contact materials heavily depends on the surface reactivity of the
materials in contact. For unknown material, determination of right
combination is impossible without heavy experimenting. For example,
Indalloy Corporation provides a research soldering kit, targeting
soldering non-standard metals. The number of alloys in the kit is
above 280, which go in combination with 5 different fluxes. This
gives more than 1400 combinations to test. Even when a mechanically
strong soldered joint is made, there is absolutely no guarantee
that it would give suitable contact resistance. The same is true
with different approaches, used for contact making. These include,
but are not limited to, different variants of vacuum evaporation
(thermal, plasma, reactive magnetron sputtering, laser ablation).
During contact making, new binary and ternary compounds are easily
formed at the interface, which adds to the poor predictability of
the contact electrical performance.
SUMMARY OF THE INVENTION
[0010] The present invention provides in an embodiment a method for
making one or more low resistivity contacts to iron pnictide
superconductors, such as for purposes of illustration and not
limitation, those based on doped AFe.sub.2As.sub.2, where A can be
Ca, Sr, Ba, Eu, or combinations thereof, as well as the electrical
contact produced. The electrical contact is characterized by low
surface contact resistivity suitable for use in both low and high
current applications.
[0011] In an illustrative embodiment of the invention, an
electrical conductor is connected to a surface of an iron pnictide
superconductor using tin (Sn) or a tin-based material, such as a
tin solder or tin-based solder, to provide a low resistivity
electrical contact. The tin-based material can comprise a
tin-silver alloy, a tin-copper alloy, a tin-lead alloy, or a
ternary tin-silver-copper alloy. For purposes of illustration and
not limitation, a low contact surface resistivity on the order of
10.sup.-9 ohm-cm.sup.2 can be provided by practice of the
invention.
[0012] Other advantages of the present invention will become
apparent from the following drawings taken with the following
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a graph of temperature dependence of resistance
measured using two different contact configurations. In a usual
four-probe scheme current was flowing between contacts 1 and 4, and
potential drop was measured between contacts 2 and 3, in a 2-probe
scheme current was flowing between contacts 2a and 3a, and voltage
was measured between contacts 2 and 3.
[0014] FIG. 2 is a graph of voltage measured between contacts 2 and
3 (see inset in FIG. 1) as a function of current passing between
contacts 2a and 3a. Above T, the curves are linear, reflecting
Ohm's law. Below T.sub.c the curves reveal broad plateau extending
up to the critical current I.sub.c.
[0015] FIG. 3 is a zoom (enlarged) view of the V-I curve measured
in the 2-probe configuration, of the data of FIG. 2. Due to the
contact resistance, voltage measured in the 2-probe scheme, is
finite and obeys Ohm's laws with contact resistance R=8
.mu..OMEGA., while the 4-probe measurements detect zero within
experimental scatter value.
[0016] FIGS. 4a and 4b are photographs showing experimental set-up
used in contact resistance measurements where current was fed
through either the current contacts at the ends of the
superconductor sample (FIG. 4a) or potential contacts (FIG. 4b).
The areas of the two contacts are A.sub.1 and A.sub.2: Since
contacts are series-connected, measured R.sub.c=R.sub.1+R.sub.2.
With R.sub.1=.rho..quadrature./A.sub.1 and
R2=.rho..quadrature./A.sub.2, it is not difficult to show that
.rho..quadrature.=A.sub.1A.sub.2/(A.sub.1+A.sub.2) where
.rho..quadrature. is contact surface area resistivity.
DETAILED DESCRIPTION OF THE INVENTION
[0017] An illustrative embodiment of the invention provides a
method for making one or more low resistivity electrical contacts
to iron pnictide superconductors which are represented by general
formula
(A.sub.1-xD1.sub.x)(Fe.sub.1-yD2.sub.y).sub.2(As.sub.1-zD3.sub.z).sub.2
wherein A comprises an alkali earth element selected from the group
consisting of Ca, Sr, Ba, Eu, and combinations thereof and x is 0
to 1, y is 0 to 0.4 where y can be 0 to 0.2 in some particular
embodiments, and z is 0 to 0.6. The element (e.g. dopant) D1 can be
selected from the group consisting of Na, K, Rb, Cs and
combinations thereof and can be present singly or collectively (if
more than one is present) in an amount up to 100 atomic % on the
alkali earth site of the superconductor material. Dopant D2 can be
selected from the group consisting of Co, Ni, Pd, Rh, Ru, Pt and
combinations thereof and can be present singly or collectively (if
more than one is present) in an amount up to 20 atomic % on the
iron site of the superconductor material. The element (dopant) D3
can be selected from the group consisting of P, Te, S, Se, Sb, Bi
and combinations thereof and can be present singly or collectively
(if more than one is present) in an amount up to 60 atomic % on the
pnictide site of the superconductor material. The contact can be
made from a tin-based alloy having a majority (greater than 50% by
weight) of tin and that can include a minor amount of an element
selected from the group consisting of Bi, Al, In, Pb, Ag, Au, Cu,
Cd, Sb, and Zn or their combinations to improve the mechanical or
soldering properties of the electrical contact and to improve a
joint to the metallic conductor.
[0018] In a particular illustrative embodiment of the invention,
the superconductor is represented by doped AFe.sub.2As.sub.2 with
the general formula
(A.sub.1-xD1.sub.x)(Fe.sub.1-yD2.sub.y).sub.2(As.sub.1-zD3.sub.z)-
.sub.2, where A can be selected from the group consisting of Ca,
Sr, Ba and Eu and combinations thereof, as well as the electrical
contact produced, wherein an electrical contact is produced having
low surface contact resistivity suitable for use in both low and
high current applications.
[0019] The electrical conductor comprises a metallic electrical
conductor which can include, but is not limited to, a metallic
wire, metallic strip, metallic strap or other shape of an
electrical conductor. The electrical conductor can include, but is
not limited to, metallic conductors comprising Ag, Ag alloys, Cu,
Cu alloys, Au, and Au alloys, and Al and Al alloys.
[0020] Pursuant to practice of the invention, the electrical
conductor and the superconductor are electrically connected using
tin (Sn) or a tin-based material, such as tin solder or tin-based
solder wherein a tin-based solder material comprises a majority
(i.e. 50% or more by weight) of tin in its composition. Ultrahigh
purity tin solder comprising 99.999% by weight tin can be used in
practice of the invention to provide a low resistivity electrical
contact. Also, tin-based materials comprising an alloy of tin and
lead, an alloy of tin and silver, and an alloy of tin and copper,
and ternary alloys can be used in practice of the invention to
provide a low resistivity electrical contact. For purposes of
illustration and not limitation, a Sn--Ag solder having 3.5 weight
% Ag and balance Sn can be used in practice of the invention. Also,
a Sn--Pb solder having 37 weight % Pb and balance Sn can be used in
practice of the invention. An illustrative ternary Sn--Ag--Cu
solder alloy is described below.
[0021] In practice of the invention wherein the electrical
conductor and a surface of the superconductor are connected by a
tin or tin-based solder, a suitable soldering flux can be used
which will permit a low resistivity electrical contact to be made.
Suitable solder fluxes include, but are not limited to,
commercially available Castolin 157 flux, Indalloy 2 flux and
HCl.
[0022] The following examples are offered to illustrate, but not
limit, embodiments of the present invention. In particular,
electrical contacts to doped AFe.sub.2As.sub.2 were made using
soldering techniques. A number of different solder alloys and
fluxes summarized in Table 1 were tested and the measured surface
resistivity .rho..quadrature. of the contacts (given as the contact
resistance R divided by the contact area) are set forth in Table
1.
[0023] FIG. 1 shows the temperature dependence of electrical
resistance measured on the sample of
Ba(Fe.sub.0.926Co.sub.0.074).sub.2As.sub.2, in which
superconductivity with T.sub.c=23K was induced by partial
substitution of Fe by Co. The contacts were made by soldering
silver wires to the clean surface of the
Ba(Fe.sub.0.926Co.sub.0.074).sub.2As.sub.2 single crystal with the
help of high purity (99.99%) tin solder. Contact resistance was
measured in two ways. First, a 4-probe configuration (FIG. 1) was
used, with current flowing between contacts 1-4 and potential drop
measured between contacts 2-3. Second, current was flowing through
the contacts 2a and 3a, while potential was measured between
contacts 2 and 3. The difference between these two measurements is
caused by the fact that in the 2-probe measurement, potential
difference includes voltage drop in the contact due to finite
contact resistance. As can be seen, the two measurements give
almost identical results, beyond the resolution of this experiment,
set by a slight difference of current distribution in two sets of
measurements. The contact resistance is shown to be definitely
below 100 .mu..OMEGA.. However, because of its very small value it
was difficult to measure it precisely in this geometry.
[0024] To obtain a better measurement of contact resistance, V-I
(voltage vs. current) characteristics at temperatures below Tc of
the compound were measured. Because of the zero resistance of the
superconductor sample, finite voltage is generated when making
measurements in two-probe technique, due to the resistance of the
two contacts. In FIG. 2 a set of V-I curves taken in 2-probe
configuration in isothermal conditions is shown. At temperatures
above T.sub.c, the curves are perfectly linear, as expected from
Ohms law. Below T.sub.c the curves reveal nonlinearity due to
approach of the critical current density J.sub.c. Since critical
current density increases on cooling, at 20.1 K (2K below Tc), the
measurement setup is not able to supply high enough current to
reach J.sub.c. Zoom of the curve at 20.1 K, FIG. 3, shows that
actually the measured 2-probe resistance, though small, causes
linear V-I curve with finite slope (corresponding to the contact
resistance R.sub.c=8 .mu..OMEGA.), as opposed to zero resistance as
detected in four-probe measurements. This is due to the resistance
of two contacts at the sample ends.
[0025] In FIG. 4a, 4b, photographs of two superconductor samples
measured in two-probe measurements between either outer contact
pair (FIG. 4a) or central contact pair (FIG. 4b). Since two
contacts are series connected, measured resistance R.sub.c is the
sum of two contact resistances, R.sub.1 and R.sub.2. Taking contact
areas of contacts 1 and 2 as A.sub.1 and A.sub.2, we come to the
following relation:
.rho..quadrature.=R.sub.cA.sub.1A.sub.2/(A.sub.1+A.sub.2). The
experimentally determined values for different contact preparation
techniques are summarized in Table 1 where "wire" is the electrical
conductor and "alloy" is the solder alloy.
[0026] As can be seen in Table 1, use of pure tin solder and of tin
alloy solders with Ag, Cu, Ag--Cu and Pb give contact resistivities
in the nano.OMEGA.-cm.sup.2 range (10.sup.-9 .OMEGA.-cm.sup.2),
comparable to the best examples known for other superconductors, as
discussed below.
Sample Preparation for Table 1 Examples
[0027] Single crystals of Ba(Fe.sub.1-yCo.sub.y).sub.2As.sub.2 were
grown out of self flux using conventional high temperature solution
growth techniques described in N. Ni et al. Phys. Rev. B 78, 214515
(2008), which is incorporated herein by reference. Small Ba chunks,
FeAs powder and CoAs powder were mixed together according to the
ratio Ba:FeAs:CoAs=1:4y(1-y):4y. Crystal dimensions can go up to
12*8*1 mm.sup.3.
Sample Characterization:
[0028] Elemental analysis of the samples was performed using
wavelength dispersive x-ray spectroscopy (WDS) in the electron
probe microanalyzer of a JEOL JXA-8200 Superprobe. According to
elemental analysis these samples have y=0.074, while load during
sample growth was y=0.10. They are characterized by
T.sub.c=22.5K.
[0029] Critical current measurements and resistivity measurements
were performed using a Quantum, Design (QD) Physical Property
Measurement System (PPMS).
[0030] Samples were cleaved into rectangular prisms, as shown in
FIG. 4a, 4b. Four contacts were arranged in a usual four-probe
contact configuration. Four contacts to the samples were made using
silver wires attached to the sample using different combinations of
soldering alloys and fluxes. In all cases to achieve reproducible
results, the contacts were put on a fresh cleaved surface of single
crystals of Ba(Fe.sub.1-yCo.sub.y).sub.2As.sub.2. Contacts with
reasonably low surface resistivity could be made with several
soldering alloys based on Sn or ultrahigh purity Sn.
TABLE-US-00001 TABLE 1 Characteristics of the electrical contacts
to doped AFe.sub.2As.sub.2 as a function of preparation conditions.
No Dopant R.sub.c, 20 Compound type, K A.sub.1 A.sub.2
.rho..quadrature. A x or y Wire Alloy Flux .OMEGA. cm.sup.2
cm.sup.2 .OMEGA.-cm2 Reference I Ba Co, Ag Sn Castolin 1.76* 1.73*
1.9* 1.59* s9151 0.074 99.999% 157 10.sup.-6 10.sup.-3 10.sup.-3
10.sup.-9 2 Ba Co, Ag Silver No 0.404 2.76* 2.49* 5.24* s9148 0.074
epoxy 10.sup.-3 10.sup.-3 10.sup.-4 3 Co, Ag Au Ag 0.1 3* 3.23*
5.24* s005 0.074 Evaporation paint 10.sup.-3 10.sup.-3 10.sup.-4 4
Ba Co, Ag Indalloy 3 Castoline 7.27* 9.9* 9.8* 4.58* s9142 0.074
157 10.sup.-6 10.sup.-4 10.sup.-4 10.sup.-9 In-10% Ag 5 Ba Co, Ag
Indalloy 3 Indalloy 1 2.41* 1.23* 4.92* 2.37* s9143 0.074
10.sup.-s4 10.sup.-3 10.sup.-4 10.sup.-8 In-10% Ag 6 Ba Co, Ag
Indalloy 3 HCl 4.76* 2.47* 3.95* 7.27* s9143b 0.074 Concentrated
10.sup.-4 10.sup.-4 10.sup.-4 10.sup.-8 In-10% Ag 7 Ba Co, Ag
Indalloy 3 Indalloy 2 Bad s9144 0.074 In-10% Ag 8 Ba Co, Ag
Indalloy 3 Indalloy 3 Bad 0.074 In-10% Ag 9 Ba Co, Ag Indalloy 3
Indalloy 4 Bad s9143d 0.074 In-10% Ag 10 Ba Co, Ag Indalloy 3
Indalloy Bad s9143c 0.074 5A In-10% Ag 11 Ba Co, Ag Indalloy
Indalloy 2 7.05* 1.03* 1.19* 3.89* s9145 0.074 121 10.sup.-6
10.sup.-3 10.sup.-3 10.sup.-9 Sn-Ag 12 Ba Co, Ag Indalloy Castolin
7.73* 8.7* 1.30* 4.03* s9149 0.074 121 157 10.sup.-6 10.sup.-4
10.sup.-3 10.sup.-9 Sn-Ag 13 Ba Co, Ag Sn 63% Castolin 8.1* 2.6*
3.41* 1.19* s9146 0.074 Pb 37% 157 10.sup.-6 10.sup.-3 10.sup.-3
10.sup.-8s 14 Ba Co, Ag Sn 63% Indalloy 2 9.98* 1.7* 1.76* 8.69*
s9147 0.074 Pb 37% 10.sup.-6 10.sup.-3 10.sup.-3 10.sup.-9 15 Ba K,
Ag Indalloy Castolin 9.93* 5.1* 6.4* 2.81* s9150 0.30 121 157
10.sup.-5 10.sup.-4 10.sup.-4 10.sup.-8 16 Ba Ni, Ag Indalloy
Castolin 4.23* 2.19* 4.03* 6*10.sup.-7 s9153 0.05 121 157 10.sup.-4
10.sup.-3 10.sup.-3 17 Ba Co, Ag In Castolin 1.85* 1.19* 1.19* 1.1*
s9152 0.074 99.99% 157 10.sup.-4 10.sup.-3 10.sup.-3 10.sup.-7 18
Ba Co, Ag Sn 97.5% Castolin 5.7* Average 0.074 Al 2.5% 157
10.sup.-9 3 samples 19 Ba Co, Ag Sn 57% Castolin 7.2* Average 0.074
Bi 43% 157 10.sup.-8 3 samples 20 Ba Co, Ag Sn 99% Castolin 3.1*
Average 3 0.074 Cu 1% 157 10.sup.-9 samples 21 Ba Co, Ag Sn 99%
Castolin 1.58* S10571 0.074 Dy 1% 157 10.sup.-8 22 Ba Co, Ag Sn 99%
Castolin 5.8* Average 2 0.074 Ti 1% 157 10.sup.-9 samples 23 Ba Co,
Ag Sn 85% Castoline 10.sup.-7 Slightly 0.074 Zn 15% 157 rectifying
contacts 24 Ba Co, Ag Ag 4.7% 1.4* Average 3 0.074 Cu 1.7%
10.sup.-9 samples Sn- 93.6% ternary alloy
[0031] In Table 1, "reference" is sample number and % is weight %
for the alloys under Alloy.
[0032] The contact resistance thus obtained was in a range of
several .mu..OMEGA. and surface resistivity of 1 to 10*10.sup.-9
.OMEGA.-cm.sup.2.
Example 1 of Table 1
[0033] Pure 99.999% tin was used as solder pursuant to the
invention and Castoline 157 eutectic flux was used as a flux.
Sample surface was covered with acidic flux and soldering was
performed by heating sample to a melting point of the Sn. The melt
showed very good wetting of the surface and the contact resistivity
of the contacts was 1.59*10.sup.-9 .OMEGA.-cm.sup.2.
Example 11 of Table 1
[0034] A commercially available Sn--Ag solder alloy (Indalloy #
121) was used pursuant to the invention in combination with
Indalloy #2 flux. The alloy was heated to a temperature between
liquidus and solidus lines of the composition. The contact
resistivity was 3.89*10.sup.-9 .OMEGA.-cm.sup.2.
Example 12 of Table 1
[0035] A commercially available Sn--Ag solder alloy (Indalloy #
121) was used pursuant to the invention in combination with
Castolin 157 flux. The alloy was heated to a temperature between
liquidus and solidus lines of the composition. The contact
resistivity was 4.03*10.sup.-9.
Example 20 of Table 1
[0036] A eutectic Sn--Cu alloy was used pursuant to the invention
in combination with Castolin 157 flux. The alloy was heated to a
temperature between liquidus and solidus lines of the composition.
The contact resistivity was 3.1*10.sup.-9 .OMEGA.-cm.sup.2.
Example 24 of Table 1
[0037] A patented eutectic Sn--Cu--Ag alloy (Ag-4.7 wt %-Cu-1.7 wt
%-Sn-93.6 wt % Sn) was used pursuant to the invention in
combination with Castolin 157 flux. The ternary alloy was heated to
a temperature between liquidus and solidus lines of the
composition. The contact resistivity was 1.4*10.sup.-9
.OMEGA.-cm.sup.2.
[0038] Other commercially available tin-based solder alloys known
as Sn63Pb37 (37 weight % Pb and balance Sn) were used pursuant to
the invention and produced results which were not too different
from using pure tin solder (see Examples 14 and 15 of Table 1).
Example 15 of Table 1
[0039] To test if the soldering technique works with compounds in
which superconductivity is induced by different dopants, contact
resistance of the contacts made following procedures of Example 12
on samples of (Ba.sub.1-xK.sub.x)Fe.sub.2As.sub.2 were tested
pursuant to the invention. The results are shown in Table 1 as
Example 15. Contact resistivity was 2.81*10.sup.-9
.OMEGA.-cm.sup.2. In part this value can be determined by the
difference in the initial status of the surface, which was not
cleaved but rather degraded due to exposure to air.
[0040] In another doping example, samples of
Ba(Fe.sub.1-yNi.sub.y).sub.2As.sub.2 were soldered pursuant to the
invention following procedure of Example 12. Contact resistivity
was determined as 6*10.sup.-9 .OMEGA.-cm.sup.2.
[0041] The Examples shown above illustrate that tin and tin-based
alloys provide very low surface resistivity contacts to the
superconductors of the
(A.sub.1-xD1.sub.x)(Fe.sub.1-yD2.sub.y).sub.2(As.sub.1-zD3.sub.z).sub-
.2 type described above.
[0042] Although the present invention has been described in
connection with certain illustrative embodiments, those skilled in
the art will appreciate that changes and modifications can be made
therein within the scope of the invention as set forth in the
appended claims.
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